The present invention relates to a lens unit, and more particularly, to a lens unit suitable for use in a compact image capturing device. Further, the present invention relates to an image capturing device.
Most of image capturing devices that are presently being used are digital cameras, which employ charged coupled device (CCD) image sensors or complementary metal oxide semiconductor (CMOS) image sensors as image capturing elements. Thus, such an image capturing device must use a lens unit of which capacities differ from that of a so-called silver salt camera. For example, the sensitivity to diagonal incident light is lower in a CCD image sensor or CMOS image sensor than in a silver salt film. Thus, a CCD image sensor or CMOS image sensor must have a small chief ray angle (CRA), which is the incident angle of light rays entering an image capturing device. Due to the improvements made on the integration of image capturing elements, the image quality must be improved even for image capturing devices used in compact cameras or mobile phones. At the same time, such an image capturing device must be further reduced in size. Furthermore, an image capturing device that is compact and used in a compact camera or mobile phone must have a structure that lowers manufacturing costs so that relatively inexpensive products can be supplied.
To improve the image quality, aberrations must be lowered. Thus, instead of a lens unit having a three-lens structure, which may easily be designed to be compact, it is desirable that a lens unit having a four-lens structure be used, which more easily suppresses aberrations. Japanese Laid-Open Patent Publication Nos. 2002-228922, 2006-309043, and 2007-11237 describe compact lens units having four-lens structures. The four-lens structures of the publications will be discussed below.
Japanese Laid-Open Patent Publication No. 2002-228922 describes a so-called front stop lens system in which the stop is located further toward an image capturing side (hereinafter referred to as the “object side”) from all the lenses of the lens unit. In such a structure, it is generally known that when misalignment occurs in a lens located toward an image capturing element side (hereinafter referred to as the “image side”) from the stop, the misalignment greatly affects the image quality. As a result, there is a tendency for product defects to occur due to such misalignment. This lowers the manufacturing yield of the lens unit and increases manufacturing costs.
Japanese Laid-Open Patent Publication No. 2006-309043 describes a so-called rear stop lens system in which the stop is located rearward from two object side lenses. It is generally known that such a structure increases the CRA. However, as described above, the lens unit for a digital camera must have a small CRA. Thus, the refractive index of each lens must be adjusted to decrease the CRA. However, if the thickness of each lens were to be increased to adjust the refractive index, this would increase the length of the entire lens unit and thereby enlarge the lens unit.
Japanese Laid-Open Patent Publication No. 2007-11237 describes a so-called middle stop lens system in which a stop is arranged between the two lenses located closest to the object, that is, between the first and second lenses from the object side. It is generally known that a misalignment would less affect the image quality in comparison to a front stop lens system and have a smaller CRA than a rear stop lens system.